1. Field of the Invention
This invention relates to a hydraulic pilot operation circuit for remote-controlling, by means of a hydraulic pilot pressure output from a remote control valve, a hydraulic control valve which controls the operation of driving a hydraulic actuator for use in a construction machine such as a hydraulic power shovel. The present invention also relates to a valve for quickly discharging an oil (referred to as a hydraulic pressure quick discharge valve hereinafter) suitable for improving the change-over response of the hydraulic control valve, the hydraulic pressure quick discharge valve being disposed, for example, in a line which connects the remote control valve and a pilot port of the hydraulic control valve to each other.
2. Description of the Prior Art
FIG. 4 shows a conventional hydraulic pilot operation circuit for controlling a change-over of a directional control valve which is a hydraulic control valve, by a hydraulic pressure supplied from a remote control valve.
When an operation lever 1 is operated so as to be turned in a direction R as viewed in FIG. 4, the pressure of oil ejected from a hydraulic pump 2 for operating-system is reduced at a rate in proportion to the amount of operation of a remote control valve 3, introduced via a line 4 to a pilot port 5a of a directional control valve 5 which is changed over by a hydraulic pilot pressure, so that the directional control valve 5 is changed from a position B (neutral position) to a position A. As a result, pressurized oil is supplied from a hydraulic pump 6 for cylinder driving system to a rod chamber of a hydraulic cylinder 7 so that a piston rod 7a retracts. When the operation lever 1 is returned to the neutral position while the directional control valve 5 is maintained at the position A after changing-over thereto, the line 4 communicates with a tank 9 via the remote control valve 3 and a line 8. The directional control valve 5 is changed to the position B by a force of a return spring 5c, thereby stopping the movement of the piston rod 7a. During this sequence of operations, the other pilot port 5b of the directional control valve 5 communicates with the tank via a line 10, a remote control valve 11, and the line 8. When the operation lever 1 is turned in a direction L, each component operates as described above, thereby extending the piston rod 7a.
If the hydraulic pilot operation circuit shown in FIG. 4 is applied to a large sized construction machine, the length of each of the lines 4 and 10 connecting the remote control valves 3 and 11 to the pilot ports 5a and 5b of the directional control valve 5 is increased, so that the resistance to an oil flowing through each of the lines 4 and 10 increases. There is therefore a possibility of occurrence of a response retardation in returning of the directional control valve 5 to the neutral position from the position A or C.
If the operation lever 1 is gradually returned to the neutral position, the directional control valve 5 is gradually changed to the neutral position generally in response to change in the pressure output from the remote control valve 3 or 11. In this case, there is no problem in terms of response retardation. If the operation lever 1 is rapidly operated when returned to the neutral position, returning of the directional control valve 5 to the neutral position is retarded. That is, the pressure output from the remote control valve 3 abruptly decreases to the tank pressure in response to an abrupt operation of the operation lever 1, so that the directional control valve 5 starts returning to the neutral position by a force of a return spring 5c. During returning of the directional control valve 5 to the neutral position, oil is discharged from the pilot port 5a (5b) into the line 4 (10) as a spool of the directional control valve 5 moves, and this oil flows to the tank via the remote control valve 3 (11) and the line 8. If the resistance to the oil flowing through the line 4 (10) undesirably increases due to increase of the length of the line 4(10) as described above, returning of the directional control valve 5 to the neutral position is delayed from returning of the operation lever 1 to the neutral position, thus deteriorating response characteristics.
When the directional control valve 5 is changed from the neutral position to the position A (C), oil which is discharged from the pilot port 5b (5a) of the directional control valve 5 into the line 10 (4) in response to the movement of the spool flows to the tank via the remote control valve 11 (3) and the line 8. Therefore, changing over the directional control valve 5 from the neutral position to the position A (C) is retarded even if the operation lever 1 is rapidly operated so as to be changed from the neutral position to the desired operative position. Thus, there is a problem of the directional control valve 5 of not being changed over in compliance with operation of the operation lever 1. The problem of response retardation relating to these two operations is considerable especially in a cold district. It is desirable to improve response characteristics in changing-over the directional control valve 5.